The invention relates to a reflective optical element, in particular for a microlithographic projection exposure apparatus or for a mask inspection apparatus.
Microlithography is used for producing microstructured components, such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (=reticle) illuminated with the illumination device is in this case projected by the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Mask inspection apparatuses are used for inspecting reticles for microlithographic projection exposure apparatuses.
In projection lenses or inspection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, reflective optical elements are used as optical components for the imaging process.
One problem that occurs in practice is that reflective optical elements designed for operation in the EUV, in particular owing to the absorption of the radiation emitted by the EUV light source, experience heating and an accompanying thermal expansion or deformation, which can in turn result in an impairment of the imaging properties of the optical system. This is the case particularly if use is made of illumination settings having comparatively small illumination poles (e.g. in dipole or quadrupole illumination settings), in which the element heating or deformation varies greatly across the optically effective surface of the reflective optical element.
Transferring solution approaches known for VUV lithography systems (having an operating wavelength e.g. of approximately 200 nm or approximately 160 nm) for overcoming the above-described problem of element heating to EUV systems has proven difficult. This is so in part because the number of optically effective surfaces available for active deformation compensation is, relative to VUV systems, greatly limited owing to the comparatively smaller number of optical elements or mirrors that are used in EUV lithography. (The number of elements or mirrors is kept small in order to avoid excessively high light losses on account of the necessary reflections).
In order to overcome the above-described problem of element heating in EUV systems it is known, in particular, to use additional devices for realizing rigid-body movements and/or temperature changes in the region of the optically effective surface of the reflective optical elements designed for operation in the EUV. Such solutions, however, increase the complexity of the systems.